Cu2XSnY4 Nanoparticles

ABSTRACT

Materials and methods for preparing Cu 2 XSnY 4  nanoparticles, wherein X is Zn, Cd, Hg, Ni, Co, Mn or Fe and Y is S or Se, (CXTY) are disclosed herein. The nanoparticles can be used to make layers for use in thin film photovoltaic (PV) cells. The CXTY materials are prepared by a colloidal synthesis in the presence of labile organo-chalcogens. The organo-chalcogens serves as both a chalcogen source for the nanoparticles and as a capping ligand for the nanoparticles.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 14/210,012filed Mar. 13, 2014, and claiming the benefit of U.S. ProvisionalApplication Ser. No. 61/799,465, filed Mar. 15, 2013. The entirecontents of both of these applications are incorporated herein byreference in their entireties.

FIELD OF THE INVENTION

The disclosure relates to materials (and processes for the preparationthereof) useful for the solution-phase fabrication of photovoltaic (PV)devices. More specifically, the disclosure describes a simple, scalable,low temperature colloidal method of synthesising Cu₂XSnY₄ nanoparticles,where X is a d-block metal and Y is a chalcogen, for potentialapplication in thin film optoelectronic devices.

BACKGROUND

In recent years, Cu(In,Ga)Se₂ (CIGS) materials have been extensivelystudied for use as an absorber layer in thin film photovoltaic devices,owing to their band gaps that can be tuned by adjusting the elementalratios and are well matched with the solar spectrum (1.1 eV for CuInSe₂to 1.7 eV for CuGaSe₂), offering potentially high conversionefficiencies; 20.3% conversion efficiency was achieved usingCu(In_(x)Ga_(1-x))Se₂ material by researchers at ZSW and the Centre forSolar Energy and Hydrogen Research in Germany (August 2010). Onedrawback of CIGS materials is the high manufacturing cost, due to thehigh cost of the constituent elements. Cu₂ZnSnS₄ (CZTS) materials can beused as a low-cost alternative to traditional Cu(In,Ga)Se₂, due to theabundance and low toxicity of Zn and Sn, which are much cheaper than Gaand the rarer In.

Recently there have been efforts to investigate the direct band gap ofthis material. CZTS is reported to have a band gap between 1.45 and 1.6eV [H. Katagiri et al., Appl. Phys. Express, 2008, 1, 041201; K. Ito etal., Jpn. J. Appl. Phys., 1988, 27 (Part 1), 2094; T. M. Friedlmeier etal., Proc. 14^(th) European PVSEC, Barcelona, Spain, 30 Jun. 1997, p.1242] and a high optical absorption coefficient (up to 10⁵ cm⁻¹) [G. S.Babu et al., J. Phys. D: Appl. Phys., 2008, 41, 205305], which aresimilar to those of CuInGaSe₂. The current record conversion efficiencyfor pure Cu₂ZnSnS₄ of 8.4% [B. Shin et al., Prog. Photovolt.: Res.Appl., 2013, 21, 72] shows great potential for this material.

Related compounds, where Zn is partially or completely substituted withanother d-block element, Sn is substituted with a group 14 elementand/or S is partially or entirely replaced with another chalcogen arealso of interest. Examples include Cu₂ZnSnSe₄, Cu₂ZnSnTe₄, Cu₂CdSnSe₄,Cu₂CdSnTe4, [H. Matsushita et al., J. Mater. Sci., 2005, 40, 2003]Cu₂FeSnS₄, [X. Zhang et al., Chem. Commun., 2012, 48, 4656], andCu_(2+x)Zn_(1-x)GeSe₄. [W. G. Zeier et al., J. Am. Chem. Soc., 2012,134, 7147] With the exception of Cu₂CoSiSe₄, and Cu₂NiSiSe₄, allcompounds in the series Cu₂-II-IV(S,Se)₄ (II=Mn, Fe, Co, Ni, Zn, Cd, Hg;IV=Si, Ge, Sn) have been synthesised and structurally characterised inthe bulk form, as described by Schafer and Nitsche. [W. Schafer and R.Nitsche, Mat. Res. Bull., 1974, 9, 645] Though the substitution of Zn,Sn and/or S may not necessarily offer financial advantages, thesecompounds are nevertheless attractive for optoelectronic applications asthey offer a range of thermodynamic, optical and electrical propertiesthat can be exploited. For example, Cu₂Zn_(1-x)Cd_(x)Sn(Se_(1-y)S_(y))₄is a semiconductor with a band gap that can be tuned from 0.77 eV(x=0.5, y=0) to 1.45 eV (x=0, y=1) and displays p-type conductivity. [M.Altosaar et al., Phys. Stat. Sol. (a), 2008, 205, 167] Ab initio studieson Cu₂ZnSnSe₄, Cu₂CdSnSe₄ and Cu₂ZnSnS₄ suggest that theirthermoelectric performance can be enhanced by Cu doping at the Zn/Cdsite. [C. Sevik and T.

a{hacek over (g)}in, Phys. Rev. B, 2010, 82, 045202] Other studies havefocussed on manipulation of the crystallographic phase of thesematerials [X. Zhang et al., Chem. Commun., 2012, 48, 4656] and theresultant changes in their electronic properties. [W. Zalewski et al.,J. Alloys Compd., 2010, 492, 35]

Methods to produce CIGS- and CZTS-type solar cells with high powerconversion efficiency (PCE) often employ vacuum-based deposition of theabsorber layer. Vacuum-based approaches typically offer high uniformity,which translates to a high quality film. However, the techniques arealso generally costly, with material consumption and energy usage beinghigh. Non-vacuum-based approaches are attractive in that they aretypically higher throughput processes, with a lower deposition cost. Onesuch method is a nanoparticle-based deposition approach. Nanoparticlesoffer several advantages over bulk materials for thin filmoptoelectronic applications. Firstly, a small amount of nanoparticlematerial can be dissolved or dispersed in a solvent, then printed on asubstrate, e.g. by spin coating, slit coating or doctor blading; vapourphase or evaporation techniques are far more expensive, requiring hightemperatures and/or pressures. Secondly, nanoparticles are able to packclosely, facilitating their coalescence upon melting. Upon coalescencethe particles can form large grains. Additionally, the melting point ofnanoparticles is lower than that of the bulk material, allowing lowerprocessing temperatures for device fabrication. Finally, nanoparticlescan be synthesised in colloidal solutions. Colloidal nanoparticles maybe capped with an organic ligand (capping agent); this assists insolubilising the particles, thus facilitating the processability of thematerial.

Nanoparticles can be synthesised from a top-down or a bottom-upapproach. In a top-down approach, macroparticles are processed, e.g.using milling techniques, to form nanoparticles; the particles aretypically insoluble, therefore difficult to process, and in the case ofmilling the size distribution may be large. Using a bottom-up approach,whereby nanoparticles are grown atom-by-atom, smaller particles with ahomogeneous size distribution may be produced. Colloidal syntheses canbe employed to grow nanoparticles in solution, which can be passivatedwith organic ligands to provide solubility, and thus solutionprocessability.

The colloidal synthesis of CZTS nanoparticles has been described in theprior art. The colloidal synthesis of Cu₂XSnY₄ nanoparticles, where X isa d-block element and Y is a chalcogen, herein referred to as “CXTY”, isless well documented, however a number of examples exist.

Ou et al. described the synthesis of Cu₂ZnSn(S_(x)Se_(1-x))₄nanoparticles via the hot-injection of a solution of Cu, Sn and Znstearate salts, dissolved in oleylamine, into a mixture of thiourea,oleylamine and octadecene, at 270° C. [K.-L. Ou et al., J. Mater. Chem.,2012, 22, 14667]

The hot-injection synthesis of Cu₂Zn_(x)Sn_(y)Se_(4x+2y) nanoparticleshas been described by Shavel and co-workers. [A. Shavel et al., J. Am.Chem. Soc., 2010, 132, 4514] Trioctylphosphine selenide was injectedinto a solution of Cu, Zn and Sn salts dissolved in a mixture ofhexadecylamine and octadecene, at 295° C.

The preparation of Cu₂FeSnS₄ nanoparticles has been described by Zhanget al. [X. Zhang et al., Chem. Commun., 2012, 48, 4656]A mixture of1-dodecanethiol and t-dodecanethiol was injected into a solution of Cu,Fe and Sn salts in oleylamine at moderate temperature (150° C.). Thesolution was subsequently heated 210° C. to prepare wurtzitenanocrystals. To synthesise zinc blende nanocrystals, the solution washeated to 310° C., replacing oleylamine with oleic acid and octadecene.

The colloidal methods of making CXTY nanoparticle materials described inthe prior art have one or more disadvantages including the use ofhot-injection and/or high boiling capping agents (ligands).

Hot-injection techniques can be used to synthesis small nanoparticleswith a uniform size distribution. The technique relies on the injectionof small volumes of precursors into a large volume of solvent atelevated temperature. The high temperature causes breakdown of theprecursors, initiating nucleation of the nanoparticles. However, thetechnique results in low reaction yields per volume of solvent, thusmaking the reactions difficult to scale to commercial volumes.

Other prior art techniques utilise high boiling ligands, such asoleylamine, hexadecylamine or oleic acid. Organic ligands assist insolubilising the nanoparticles to facilitate solution processability,yet they must be removed, e.g. by evaporation, prior to sintering, sinceresidual carbon can be detrimental to the optoelectronic performance ofthe absorber layer. Thus it is favourable that the boiling temperatureof any capping ligand(s) should be substantially lower than thesintering temperature of the CXTY film.

Thus, there is a need for a commercially scalable method to synthesiseCXTY nanoparticle capped with a relatively low-boiling ligand that issuitable for low temperature optoelectronic device processing.

BRIEF SUMMARY

Materials and methods for preparing Cu₂XSnY₄ (CXTY) nanoparticles,wherein X is Zn, Cd, Hg, Ni, Co, Mn or Fe and Y is S or Se, aredisclosed herein. The nanoparticles can be used to make layers for usein thin film PV cells. The CXTY nanoparticles are prepared by acolloidal synthesis. The disclosed methods are advantageous over theprior art because they are scalable for mass manufacture (kg scale) ofPV materials. The scalability is due to a high yield per volume ofreaction solution.

For thin film photovoltaic applications, the organic ligand-cappednanoparticles are dissolved or dispersed in solution, then deposited ona substrate using a printing or coating technique. Prior to sintering,the ligand must be removed by annealing within the device processingconditions to remove carbon from the film. As such, the ligand ispreferably labile. The processes described herein provide labileligand-capped nanoparticles, i.e., nanoparticles for which the ligand iseasily removed at moderate temperatures. The process involves reactingcopper precursors, X (as defined above) precursors, and Sn precursors inthe presence of labile organo-chalcogens. The organo-chalcogens serveboth as a source of chalcogen (i.e., S or Se) for the nanoparticlematerial and as the labile surface-capping ligands.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a flow chart summarising the process for preparing CXTYnanoparticles according to the method described herein.

FIG. 2 is a flow chart summarising the process for preparing a PV devicefrom CXTY nanoparticles according to the method described herein.

FIG. 3 shows the X-ray diffraction (XRD) pattern for CZTSe nanoparticlesprepared according to Example 1.1, which matches well with the CZTSestannite and Cu₂Se₃ umangite phases. Wurtzite ZnSe [K. Yvon et al., J.Appl. Cryst., 1977, 10, 73], a common impurity in CZTSe materials isincluded for reference, but does not appear to exist in this material.

FIG. 4 shows thermogravimetric analysis (TGA) of CZTSe nanoparticlescapped with 1-octane selenol, prepared according to Example 1.1, showingan inorganic content of 65%.

FIG. 5 shows the XRD pattern of CZTSe nanoparticles synthesised with1-dodecane selenol, as described in Example 1.2. The peak positionsmatch well with the wurtzite crystal structure of ZnSe. [K. Yvon et al.,J. Appl. Cryst., 1977, 10, 73] Differences in the relative peakintensities and slight shifting of the peak positions may beattributable to the partial substitution of Zn with Cu and Sn.

FIG. 6 is a TGA plot of CZTSe nanoparticles capped with 1-dodecaneselenol, as prepared in Example 1.2, showing removal of the ligandbetween approximately 200-320° C.

FIG. 7 is an XRD pattern of CFTS nanoparticles as prepared in Example2.1. The peak positions are well matched to those of mawsonite CFTS. [J.T. Szymanski, Canad. Mineral., 1976, 14, 529]

FIG. 8 is a TGA plot of CFTS nanoparticles capped with 1-dodecanethiol,as prepared in Example 2.1.

FIG. 9 is an XRD pattern of CZTSSe nanoparticles prepared according toExample 3.1. The peak positions are well matched to those of stanniteCZTSe. [Olekseyuk et al., J. Alloys Compd., 2002, 340, 141]

FIG. 10 is an XRD pattern of CCdTSe nanoparticles prepared according tomethod 4.1. The peak positions are well matched to those of stanniteCCdTSe. [Olekseyuk et al., J. Alloys Compd., 2002, 340, 141] Lowintensity, unidentified peaks suggest the presence of a minor, secondaryphase.

DESCRIPTION

Applicant's co-pending U.S. Patent Application 61/798,084 (filed 15 Mar.2013) describes the synthesis of CZTS nanoparticles capped with anorgano-thiol ligand, and is hereby incorporated by reference in itsentirety. As used herein, the term “CXTY” refers to compounds of theform Cu₂XSnY₄, where X is a d-block metal and Y is a chalcogen. It willbe understood that the formula is not meant to imply a stoichiometrywhere the ratio of Cu:X:Sn:Y is exactly 2:1:1:4; one of skill in the artwill appreciate that those ratios are estimates only. As used herein,“low temperature synthesis” refers to a heating-up method of synthesiswherein the reaction solution is heated at temperatures of 300° C. orbelow, more particularly 250° C. or below, or 240° C. or below, toeffect the conversion of the precursors to nanoparticles. In aheating-up method, the nanoparticles are combined at modesttemperatures, for example, between room temperature and 200° C., and thereaction solution is subsequently heated to induce nanoparticleformation. Thus, a low temperature synthesis is different than ahot-injection synthesis because the precursors are combined atsignificantly lower temperatures and the concentration of the precursorsin the reaction solution is significantly higher than for ahot-injection reaction.

As explained above, the surfaces of nanoparticles are generally cappedwith organic ligands that prevent the nanoparticles from agglomeratingand that aid in the solution and deposition of the nanoparticles to makefilms. Generally, prior art techniques utilise high boiling ligands,such as oleylamine. The organic ligands must be removed, e.g. byevaporation, prior to sintering, since residual carbon can bedetrimental to the performance of the absorber layer. Aspects of theprocesses described herein provide CXTY nanoparticles that havingsurface coatings consisting essentially of labile organo-chalcogenligands. As used herein, the term “labile organo-chalcogen” refers to anorgano-chalcogen with a boiling point less than 300° C. According tosome embodiments, nanoparticles having surface coatings consistingessentially of labile organo-chalcogens readily lose those cappingligands when the nanoparticles are heated to moderate temperatures.According to some embodiments, greater than 50% of the surface coatingconsisting essentially of labile organo-chalcogen is removed from thenanoparticle surface when the nanoparticle is heated to 350° C. Sincethe surface coatings provided by the methods described herein areremovable at lower temperatures than those described in the prior art,the resulting nanoparticles are more easily processed and yield higherperforming films. The nanoparticles described herein, having surfacecoatings consisting essentially of labile organo-chalcogens provide anadvantage over the CXTY nanoparticles described in the art, which havehigh-boiling capping ligands, such as oleylamine or oleic acid.

The process disclosed herein comprises a relatively low-temperature,high yield synthesis of nanoparticles of the form CXTY, where X is ad-block metal (e.g., Zn, Cd, Hg, Ni, Co, Mn or Fe) and Y is a chalcogen(e.g., S or Se). The CXTY material can be used as an alternative to CIGSmaterials as an absorber layer in thin film photovoltaic devices.

An embodiment of a process for forming CXTY nanoparticles is summarisedin FIG. 1. The process comprises: (a) combining Cu, X and Sn precursorsin a solvent at a first temperature; (b) adding an organo-chalcogenligand; (c) heating to a second temperature for a first time interval toform the CXTY nanoparticles; (d) cooling the reaction mixture; and (e)isolating the CXTY nanoparticles. Using the illustrated method, it ispossible to produce a wide range of CXTY nanoparticle material.

Suitable Cu precursors include, but are not restricted to: acetates,e.g. Cu(ac), Cu(ac)₂; acetylacetonates, e.g. Cu(acac)₂; and chlorides,e.g. CuCl, CuCl₂. X can be one or more d-block elements, including, butnot restricted to: Zn, Cd, Hg, Ni, Co, Mn, Fe. Suitable precursors mayinclude, but are not restricted to: acetates, e.g. Zn(ac)₂, Cd(ac)₂,Cd(ac)₂.2H₂O, Hg(ac)₂, Ni(ac)₂.4H₂O, Co(ac)₂.4H₂O, Mn(ac)₂.2H₂O,Fe(ac)₂; acetylacetonates, e.g. Cd(acac)₂, Ni(acac)₂, Ni(acac)₃,Co(acac)₂, Co(acac)₃, Mn(acac)₂, Mn(acac)₃, Fe(acac)₂, Fe(acac)₃;chlorides, e.g. ZnCl₂, CdCl₂, HgCl₂, NiCl₂, CoCl₂, MnCl₂, FeCl₂,FeCl₂.4H₂O, FeCl₃, FeCl₃.6H₂O; and stearates, e.g. Zn(st)₂, Ni(st)₂,Co(st)₂.

Suitable Sn precursors include, but are not restricted to, chlorides,e.g. SnCl₄, SnCl₄.5H₂O, tin(IV)acetate, tin(IV) bis(acetylacetonate)dichloride and triphenyl(trimethyl) tin. A particularly suitable Snprecursor is SnCl₄ as a solution in dichloromethane, due to its relativeease and safety in handling. The dichloromethane solvent can be removedby distillation during the nanoparticle synthesis.

The solvent is used to dissolve or suspend the Cu, X and Sn precursors.In some embodiments, the solvent is dichloromethane. Other suitableexamples include, but are not restricted to, non-coordinating solventssuch as 1-octadecene and Therminol® 66 [Eastman Chemical Company]. Oneskilled in the art will realise that, where the boiling point of thesolvent is lower than the reaction temperature required to effect theconversion of the precursors to CXTY nanoparticles, it may be necessaryto distil the solvent during the course of heating the reaction mixtureto the second (reaction) temperature.

The first temperature, at which the Cu, X and Sn precursors are combinedwith the solvent, is substantially below the boiling point of thesolvent. In some embodiments, the first temperature is room temperature.

The organo-chalcogen ligand is of the form R—Y—H, where R is an alkyl oraryl group and Y is sulphur or selenium. In some embodiments, theorgano-chalcogen ligand functions as both the reaction solvent and thenanoparticle capping agent. In some embodiments, the organo-chalcogenligand comprises two or more organo-chalcogen compounds. In particularembodiments, it may be desirable for the boiling point(s) of theorgano-chalcogen ligand(s) to fall within the range 180-300° C., toallow both synthesis of phase-pure CXTY nanoparticles and to effect theremoval of the ligand(s) during subsequent device processing. Examplesof suitable organo-chalcogen ligands include, but are not restricted to:1-octanethiol, 1-dodecanethiol, t-dodecanethiol, 2-naphthalenethiol,1-octane selenol, and 1-dodecane selenol.

According to some embodiments, the organo-chalcogen ligand is combinedwith the Cu, X and Sn precursors and the first solvent at a temperaturenot greater than 50° C., for example, room temperature.

In some embodiments, two or more d-block metals (X) and/or two or morechalcogens (Y) are combined to form an alloyed material.

The reaction mixture is heated to a second temperature for a first timeinterval to effect the conversion of the precursors to CXTYnanoparticles. In some embodiments, the second temperature lies in therange 180-300° C., for example around 220-240° C. The first timeinterval lies in the range of 30 minutes-5 hours, for example around 1hour.

Optionally, a secondary chalcogen precursor can be added to the reactionsolution during synthesis to maintain particle growth. Suitableprecursors include, but are not restricted to, trioctylphosphinesulphide, and trioctylphosphine selenide.

Following nanoparticle formation, the reaction mixture is cooled and theCXTY nanoparticles are isolated. The nanoparticles may be isolated viaany method known in the prior art, for example, by precipitating thenanoparticles using a combination of solvents and non-solvents, thencollecting the nanoparticles via centrifugation. Examples of suitablesolvent/non-solvent combinations include chloroform and acetone, anddichloromethane and methanol.

The CXTY nanoparticles can be processed and incorporated into aphotovoltaic device. A process preparing a thin film using CXTYnanoparticles is shown in FIG. 2. The method comprises: (a) dissolvingor dispersing the CXTY nanoparticles in one or more solvents to form anink; (b) depositing the ink on a substrate; (c) annealing at a firsttemperature, for a first time interval, under an inert atmosphere toremove the ligand; (d) annealing at a second temperature, for a secondtime interval, under an inert atmosphere to induce crystallisation ofthe film; and (e) optionally annealing at a third temperature, for athird time interval, under a chalcogen-rich atmosphere. Subsequentlayers can then be deposited to form a photovoltaic device.

The CXTY nanoparticles can be dissolved or dispersed in one or moresolvents by any method known to one skilled in the art, includingshaking, stirring or ultrasonication. In some embodiments, thesolvent(s) is/are non-polar. Examples include, but are not restrictedto, toluene, alkanes (e.g. hexane), chlorinated solvents e.g.(dichloromethane, chloroform, etc.), ketones (e.g. isophorone), ethers(e.g. anisole), and terpenes (e.g. α-terpinene, limonene, etc.).Optionally, other additives, such as binders, rheology modifiers, andthe like, may be incorporated into the ink formulation to modify itscoating properties.

The ink can be deposited on a substrate using any method known to oneskilled in the art. Examples include, but are not restricted to,spin-coating, slit-coating, drop-casting, doctor blading, and inkjetprinting.

Once deposited, the ink is annealed at a first temperature to remove thesolvent, ligand, and other organic components of the ink formulation.This eliminates carbon residues, which can be detrimental to deviceperformance, from the film. It will be apparent to one skilled in theart that the first annealing temperature depends on the boiling pointsof the organic components of the nanoparticle ink. In particularembodiments, the first annealing temperature lies in the range 260-350°C., for example around 300° C. In some embodiments, the first timeinterval preferably lies in the range 3-10 minutes, for example around 5minutes. In some embodiments, the first annealing step is conductedunder an inert atmosphere.

The films are annealed at a second temperature to induce crystallisationof the CXTY layer (sintering). In some embodiments, the second annealingtemperature is higher than the first annealing temperature. For example,the second annealing temperature may lie in the range 350-440° C., forexample around 400° C. In particular embodiments, the second timeinterval lies in the range 3-10 minutes, for example around 5 minutes.In some embodiments, the sintering step is conducted under an inertatmosphere.

The ink deposition, first and second annealing steps may be repeateduntil a desired film thickness is achieved. Optionally, the films may beannealed under a chalcogen-rich atmosphere. Examples of sulphurisationsources include, but are not restricted to, H₂S and elemental sulphur.Examples of selenisation sources include, but are not restricted to,H₂Se and elemental selenium. In some embodiments, the third annealingtemperature lies in the range 500-600° C., for example around 550° C.The third time interval may, for example, lie in the range 30 minutes-3hours, more particularly around 1-2 hours.

Additional layers can be deposited on top of the CXTY layer to form a PVdevice. The method of forming CXTY nanoparticles is illustrated in thefollowing examples.

EXAMPLES Example 1 Synthesis of CZTSe Nanoparticles Example 1.1

Synthesis of CZTSe nanoparticles using 1-octane selenol as the seleniumprecursor. 1.0 g of Cu(ac) (8.2 mmol; ac=acetate), 0.74 g of Zn(ac)₂(4.0 mmol), and 4.1 mL of a 1 M solution of SnCl₄ in dichloromethane(4.1 mmol) were stirred at room temperature in a 50 mL three-neckedround-bottomed flask fitted with a magnetic stirrer and a condenser witha side-arm. 5 mL of dichloromethane were added to dissolve/suspend thesalts, forming a grey solution. The mixture was degassed by bubblingthrough nitrogen at room temperature. After stirring under nitrogen for1% hours the solution had turned a beige/cream colour. 11.6 mL of1-octane selenol (65.0 mmol) were injected quickly into the flask; themixture turned instantly dark red. The temperature was raised to 50-55°C. and held for 5 minutes to allow the dichloromethane, which collectedin the side-arm of the condenser, to distil off. The mixture turned abright golden orange colour. The temperature was raised to 140° C.,whereupon a brown/black slurry formed. The temperature was held at 140°C. for 1 hour, before cooling to room temperature. The product, a blacksolid (1.63 g), was isolated with chloroform and acetone. The solid wascollected by centrifugation. The particles were dispersible in non-polarsolvents.

Elemental analysis by inductively coupled plasma optical emissionspectrometry (ICP-OES) gave the following elemental ratios: C, 21.44%;H, 3.78%; Cu, 22.63%; Zn, 4.50%; Sn, 8.82%; Se, 37.24%. Normalising toSn, this gives a stoichiometry of Cu_(4.79)Zn_(0.92)Sn_(1.00)Se_(6.35),suggesting that the material is Cu- and Se-rich. The selenol ligandcontributes to the total Se content. X-ray diffraction (XRD) analysis(FIG. 3) suggests the presence of both the stannite phase of CZTSe andan umangite Cu₂Se₃ impurity phase. For thin film PV applications, copperselenide impurity phases can act as a sintering flux. Afterwards, theycan be selectively removed from the absorber layer via KCN etching, [Q.Guo et al., Nano Lett., 2009, 9, 3060] leaving behind a stoichiometric,phase pure CZTSe film.

The stoichiometry can be tuned by altering the ratios of the metalprecursors. Phase purity may be achieved by controlling the reactionconditions. For example, in the colloidal synthesis of CZTSnanoparticles using metal acetate precursors, Kameyama et al. reportedthe presence of copper selenide impurity phases at low temperatures,which could be eliminated by increasing the reaction temperature above180° C. [T. Kameyama et al., J. Mater. Chem., 2010, 20, 5319] Similarly,to synthesise phase-pure CZTSe nanoparticles using the current method,1-octane selenol may be substituted for a higher boiling selenolprecursor, acting as both the reaction solvent and ligand, to facilitatea higher reaction temperature and thus preferential formation of theCZTSe phase. Suitable higher boiling selenol compounds include, but arenot restricted to, 1-dodecane selenol, as in Example 1.2, where CZTSenanoparticles with a pure wurtzite crystal structure were synthesised at240° C.

Thermogravimetric analysis (TGA, FIG. 4) shows that the material has aninorganic content of approximately 65%. The ligand is completely removedbelow 300° C., which would enable relatively low temperatureoptoelectronic device processing.

Example 1.2

Synthesis of CZTSe nanoparticles using 1-dodecane selenol as theselenium precursor. 1.00 g of Cu(ac) (8.2 mmol) and 0.74 g of Zn(ac)₂(4.0 mmol) were purged with nitrogen in a 50 mL three-neckedround-bottomed flask fitted with a magnetic stirrer and a condenser witha side-arm. 5 mL of dichloromethane and 4.1 mL of a 1 M solution ofSnCl₄ in dichloromethane (4.1 mmol) were stirred at room temperature,under nitrogen, for 90 minutes to form a beige suspension. 15 mL of1-dodecane selenol were injected quickly into the flask. The temperaturewas raised to 50-60° C. to allow the dichloromethane, which collected inthe side-arm of the condenser, to distil off. The temperature was raisedto 240° C. and held for 1 hour, before cooling to room temperature. Theproduct, a black powder (2.49 g), was isolated with chloroform/acetoneand dichloromethane/methanol, and collected by centrifugation. Theparticles were dispersible in non-polar solvents.

Elemental analysis of the inorganic content by ICP-OES gave thefollowing elemental ratios: Cu, 15.76%; Zn, 7.53%; Sn, 20.13%; Se,53.62%. Normalising to Sn, this gives a stoichiometry ofCu_(1.46)Zn_(0.67)Sn_(1.00)Se_(4.00), where the selenol ligandcontributes to the total Se content, therefore it is reasonable toassume that the material is Sn-rich. The stoichiometry can be tuned byaltering the ratios of the metal precursors. XRD analysis (FIG. 5)suggests a wurtzite crystal structure; the peak positions match wellwith those of wurtzite ZnSe. [K. Yvon et al., J. Appl. Cryst., 1977, 10,73] Slight shifting of the peak positions may be attributable to thepartial substitution of Zn with Cu and Sn. Four-coordinate Zn²⁺ has anionic radius of 60 pm, while those of Cu⁺ and Sn⁴⁺ are 46 pm and 74 pm,respectively; [C. E. Housecroft and E. C. Constable, Chemistry (3^(rd)Edition); Pearson Education Limited: Harlow, 2006; pp. 1195-1197] theslight shift of the XRD pattern of CZTSe to lower angles compared tothose of ZnSe suggests an overall expansion of the wurtzite unit cellupon the incorporation of Cu and Sn. Differences in the relative peakintensities of the CZTSe nanoparticles compared to wurtzite ZnSe maysuggest preferential crystal growth in one or more directions.

TGA indicates that the 1-dodecane selenol ligand begins to evaporate ataround 200° C. and is totally removed by around 320° C., as shown inFIG. 6. Further mass loss above 350° C. may be due to loss of inorganicmaterial, suggesting the dodecane selenol-capped CZTSe nanoparticles areparticularly suited to relatively low temperature thermal processing(<350° C.).

Example 2 Synthesis of CFTS Nanoparticles Example 2.1

Synthesis of CFTS nanoparticles using Fe(acac)₃ (acac=acetylacetonate)as the iron precursor. 1.0 g of Cu(ac) (8.2 mmol), 1.48 g of Fe(acac)₃(4.2 mmol), and 4.1 mL of a 1 M solution of SnCl₄ in dichloromethane(4.1 mmol) were stirred at room temperature in a 50 mL three-neckedround-bottomed flask fitted with a magnetic stirrer and a condenser witha side-arm. 5 mL of dichloromethane were added to dissolve/suspend thesalts, forming a dark red mixture. The mixture was degassed by bubblingthrough nitrogen at room temperature for 1% hours. 15.5 mL of1-dodecanethiol (64.7 mmol) were injected quickly into the flask; themixture turned instantly brown. The temperature was raised to 60° C. andthe dichloromethane, which collected in the side-arm of the condenser,was distilled off. The temperature was raised to 220-240° C., then heldand stirred for 1 hour, before cooling to room temperature. The product,a black powder (1.57 g), was isolated with chloroform and acetone. Thesolid was collected by centrifugation and dried under vacuum. Theparticles were dispersible in non-polar solvents.

Elemental analysis by ICP-OES gave the following elemental ratios: C,15.12%; H, 2.98%; Cu, 35.31%; Fe, 2.53%; Sn, 16.15%; S, 19.69%.Normalising to Sn, this gives a stoichiometry ofCu_(4.08)Fe_(0.33)Sn_(1.00)S_(4.51), suggesting that the material isCu-rich, and that Cu may be doping Fe vacancies and interstitialpositions. The thiol ligand contributes to the total S content. XRDanalysis (FIG. 7) shows that the material closely matches the mawsonitecrystal structure of CFTS, [J. T. Szymanski, Canad. Mineral., 1976, 14,529] without any obvious impurity phases. The stoichiometry can be tunedby altering the ratios of the precursors.

TGA (FIG. 8) indicates that the material has an inorganic content ofapproximately 72% at 600° C. Loss of the 1-dodecanethiol ligand isaccounted for by the high negative gradient of the curve between250-350° C. Further mass loss beyond 350° C. is likely to be that ofelemental sulphur from the nanoparticles (boiling point: 444.7° C.). TheTGA thus highlights that a low boiling ligand is advantageous for thinfilm optoelectronic applications, since the ligand can be removed atlower boiling temperatures without concomitant loss of inorganic sulphurfrom the nanoparticles.

Example 2.2

Synthesis of CFTS nanoparticles using Fe(acac)₂ as the iron precursor.1.0 g of Cu(ac) (8.2 mmol), 1.06 g of Fe(acac)₂ (4.2 ml), and 4.1 mL ofa 1 M solution of SnCl₄ in dichloromethane (4.1 mmol) were stirred atroom temperature in a 50 mL three-necked round-bottomed flask fittedwith a magnetic stirrer and a condenser with a side-arm. 5 mL ofdichloromethane were added to dissolve/suspend the salts, forming a darkbrown mixture. The mixture was degassed by bubbling through nitrogen atroom temperature for 1½ hours. 15.5 mL of 1-dodecanethiol (64.7 mmol)were injected quickly into the flask; the mixture remained brown. Thetemperature was raised to 60° C. and the dichloromethane, whichcollected in the side-arm of the condenser, was distilled off. Themixture was heated to 240° C. At 170° C. the mixture becameorange/brown, darkening with further increases in temperature. Thesolution was then held and stirred at 230-240° C. for 1 hour, beforecooling to room temperature. The product, a black solid (1.2 g), wasisolated with chloroform and acetone. The solid was collected bycentrifugation and dried under vacuum. The particles were dispersible innon-polar solvents, including toluene, cyclohexane, and hexanethiol.

Elemental analysis by ICP-OES gave the following elemental ratios: C,18.68%; H, 3.41%; Cu, 38.25%; Fe, 4.20%; Sn, 12.49%; S, 18.57%.Normalising to Sn, this gives a stoichiometry ofCu_(5.72)Fe_(0.71)Sn_(1.00)S_(5.50), suggesting that the material isCu-rich, and that Cu may be doping Fe vacancies and interstitialpositions. The thiol ligand contributes to the total S content. Thestoichiometry can be tuned by altering the ratios of the precursors.

Example 3 Synthesis of CZTSSe Nanoparticles

As the current method can be used to synthesise CZTSe nanoparticles andthe applicant's co-pending U.S. patent application 61/798,084 (filed 15Mar. 2013) describes the synthesis of CZTS nanoparticles, it will beobvious to one skilled in the art that by using a combination of one ormore alkyl and/or aryl thiol and one or more alkyl and/or aryl selenolcompounds, nanoparticle material of the form Cu₂ZnSn(S,Se)₄ can beproduced, as shown in Example 3.1. The stoichiometry of the material,and thus the band gap, can be tuned by controlling the thiol:selenolratio. The upper limit to the reaction temperature will be restricted bythe lowest boiling point of the organo-chalcogen compounds. Theorgano-chalcogen compounds can be mixed prior to their addition to thereaction solution, or alternatively they can be injected concurrently orsequentially.

In some embodiments, the organo-thiol compound(s) have a boilingtemperature greater than 180° C. but substantially below the boilingpoints of elemental sulphur and selenium, for example below 300° C.Suitable examples include, but are not restricted to: 1-octanethiol,1-dodecanethiol, t-dodecanethiol, 2-naphthalenethiol.

In some embodiments, the organo-selenol compound(s) have a boilingtemperature greater than 180° C. but substantially below the boilingpoints of elemental sulphur and selenium, for example below 300° C.Suitable examples include, but are not restricted to: 1-octane selenol,1-dodecane selenol.

Example 3.1

Synthesis of CZTSSe nanoparticles using 1-octane thiol and 1-octaneselenol as the chalcogen precursors. 1.00 g of Cu(ac) (8.2 mmol) and0.74 g of Zn(ac)₂ (4.0 mmol) were purged with nitrogen in a 100 mLthree-necked round-bottomed flask fitted with a magnetic stirrer and acondenser with a side-arm. 480 μL of SnCl₄ in 4 mL of dichloromethane(4.1 mmol) were added, followed by a further 5 mL of dichloromethane,then the mixture was stirred at room temperature, under nitrogen, for 1hour. 6.0 mL of 1-octanethiol (35 mmol) and 6.0 mL of 1-octane selenol(34 mmol), both pre-degassed, were injected quickly into the flask. Thetemperature was raised to 55° C. and held to allow the dichloromethane,which collected in the side-arm of the condenser, to distil off. Thetemperature was raised to 220° C. and held for 1 hour, before cooling toroom temperature. The product (2.58 g), a black powder, was isolatedwith chloroform/acetone and dichloromethane/methanol. The solid wascollected by centrifugation. The particles were dispersible in non-polarsolvents.

Elemental analysis of the inorganic components of the material byICP-OES gave the following elemental ratios: Cu, 15.99%; Zn, 6.89%; Sn,19.24%; S, 1.8%; Se, 47.28%. Normalising to Sn, this gives astoichiometry of Cu_(1.55)Zn_(0.65)Sn_(1.00)S_(0.35)Se_(3.69),suggesting that the material is slightly Sn-rich. The organo-chalcogenligands contribute to the total S and Se content. XRD analysis (FIG. 9)shows that the material closely matches the stannite crystal structureof CZTSe, [Olekseyuk et al., J. Alloys Compd., 2002, 340, 141],supporting the relatively low proportion of S to Se as determined byICP. A number of low intensity, unidentified peaks suggest the presenceof a small proportion of an impurity phase. The stoichiometry can betuned by altering the ratios of the metal precursors.

The inorganic content, as determined by TGA, was approximately 77% at600° C. The TGA profile was similar to that of the 1-octaneselenol-capped CZTSe nanoparticles in FIG. 4, with complete removal ofthe ligands below 350° C.; co-evaporation of the thiol and selenolligands is facilitated by their relatively similar boiling points.

Example 4 Synthesis of CCdTSe Nanoparticles Example 4.1

Synthesis of CCdTSe nanoparticles using 1-octane selenol as the seleniumprecursor. 1.00 g of Cu(ac) (8.2 mmol), 1.076 g of Cd(ac)₂.2H₂O (4.0mmol) and 10 mL of 1-octadecene where degassed under vacuum for 1 hourin a 100 mL three-necked round-bottomed flask fitted with a magneticstirrer and a condenser with a side-arm. The flask was purged withnitrogen. 480 μL of SnCl₄ in 4 mL of dichloromethane (4.1 mmol) wereadded, then the mixture was stirred at room temperature for 1 hour. 12.0mL of 1-octane selenol (67 mmol) were injected quickly into the flask.The temperature was raised to 55° C. and held to allow thedichloromethane, which collected in the side-arm of the condenser, todistil off. The temperature was raised to 225° C. and held for 1 hour,before cooling to room temperature. The product, a black powder (2.72g), was isolated with chloroform/acetone and dichloromethane/methanol.The solid was collected by centrifugation. The particles weredispersible in non-polar solvents.

Elemental analysis of the inorganic components of the material byICP-OES gave the following elemental ratios: Cu, 15.47%; Cd, 11.62%; Sn,17.78%; Se, 42.16%. Normalising to Sn, this gives a stoichiometry ofCu_(1.63)Cd_(0.69)Sn_(1.00)Se_(3.56), suggesting that the material isslightly Sn-rich. The inorganic content, as determined by TGA, wasapproximately 96% at 600° C., suggesting the particles are capped withrelatively little ligand. XRD analysis (FIG. 10) shows that the majorpeaks closely match the stannite crystal structure of CCdTSe. [Olekseyuket al., J. Alloys Compd., 2002, 340, 141] Low intensity, unidentifiedpeaks suggest the presence of a minor, secondary phase, possibly abinary, ternary or quaternary selenide phase. The stoichiometry can betuned by altering the relative ratios of the precursors.

1. A nanoparticle comprising: a semiconductor material having theformula Cu₂XSnY₄, where X is Zn, Cd, Hg, Ni, Co, Mn, or Fe and Y is S orSe; and, a surface coating consisting essentially of labileorgano-chalcogen ligands.
 2. The nanoparticle recited in claim 1 whereinY comprises Se when X is Zn.
 3. The nanoparticle recited in claim 1wherein the labile organo-chalcogen ligands comprise 1-octanethiol,1-dodecanethiol, t-dodecanethiol, 2-naphthalenethiol, 1-octane selenoland/or 1-dodecane selenol.